The removal selectivity of heavy metal cations in micellar-enhanced ultrafiltration: A study based on critical micelle concentration

Micellar enhanced ultrafiltration (MEUF) is a process to remove heavy metal by complexing it with the counterionic surfactant micelle, and the complex formed was subsequently removed using ultrafiltration. The surfactant is usually added in excess, which renders high rejection (low selectivity) for all metal cations in the mixture. This work emphasises MEUF conducted at surfactant dosage around the critical micelle concentration (CMC). Few studies revealed that the presence of counterions could lower the CMC of a charged surfactant. The magnitude of reduction depends on the strength of electrostatic interaction between the counterions and surfactant. Such observation suggests that the MEUF process's selectivity could be altered based on the dosage of surfactant and the types of cations presented in the solution. This work started with determining CMC of sodium dodecyl sulfate (SDS) in the presence of single or binary metal cations at varying pH (pH 3, pH 10 and without pH adjustment). The lowest CMC (4.00 mM) was obtained in the solution containing 100 ppm copper (II) at pH 3, about 42% lower compared to CMC of SDS obtained in pure water without pH adjustment (7.00 mM). Then, the filtration was performed using 7 mM of SDS, under varying pH (pH 3, pH 10 and no pH adjustment), initial metal concentration of 100 mg/L, pressure of 2.5 bar using polyethersulfone (PES) membrane of 10 k Da molecular weight cut-off (MWCO). Overall, the highest metal rejection was obtained at pH 10, with 66% of copper and 26% of chromium (III) was rejected. The highest selectivity of copper (II) over chromium (III) was observed at neutral pH. Such findings could be due to the higher diffusivity of copper cations over chromium cations in aqueous since copper cations can reach the micelle surface and form metal-micelle complex faster than chromium cations.